Add external BrAPI site management interface

An interface should be designed in order to:

• add/edit/remove external BrAPI site URLs
• forward BrAPI calls to saved sites
• agregate sites output

A new entity type "BrAPI site" should be created. For some BrAPI calls, there should be a checkbox that says "Forward this call to an other BrAPI server". When checked, a drop-down is displayed, allowing to select a BrAPI site. For some calls, it should be possible to select more than one BrAPI site.